Thermoplastics have been widely used in the formation of medical parts for which excellent design, performance, and volume capabilities are required. In order to be useful in medical applications, a thermoplastic must meet various criteria. The plastics must be inexpensive and easy to manufacture. The plastic should not react with or leach additives into physiologic solutions. The plastics must have resistance to degradation and/or deformation from steam, irradiation and ethylene oxide sterilization techniques. For use in medical tubing, plastics must have beneficial kink-resistance, coilability, contact clarity, flexibility and bondability.
One resin that has been used successfully in medical environments is poly(vinyl chloride) ("PVC"). The relevant art prior to and the subsequent development of this material as a medical plastic is reviewed in R. N. Burgess, ed., Manufacture and Processing of PVC, Macmillan Publishing Co., Inc., New York, 1982.
PVC, however, is subject to certain drawbacks and disadvantages. Most notably, PVC requires a considerable proportion of plasticizers in order to make the flexible material. Further, PVC contains a high level of halogen atoms (i.e., about 57% by weight halogen atoms). The halogen atoms pose problems for disposal of used PVC medical tubing.
With recent environmental movements to control the amount of halogen atoms in plastics, it is desirable to have medical tubing having trace or no halogen atoms. Disposal of most medical waste is usually accomplished by incineration. A halogen-free medical tubing would have no halogen atoms to pose environmental problems.
U.S. Pat. No. 3,865,776 issued to Gergen relates to kink-resistant polymeric tubing. The tubing is prepared by a combination of block copolymers, mineral white oil, polypropylene, and a resin.
U.S. Pat. No. 4,266,542 issued to Becker et al relates to ethylene vinyl acetate formulations having improved properties. The compositions consist essentially of a major portion of ethylene-vinyl acetate copolymer which contains 22% to 30% by weight of vinyl acetate units and having a melt flow index of 0.5 to 10 and a minor portion of a second ethylene-vinyl acetate copolymer having 25 to 40% by weight vinyl acetate units and having a melt flow index of 20 to 1000.
U.S. Pat. No. 4,675,364 issued to Churma et al relates to modification of ethylene vinyl acetate copolymer. The melt index of ethylene-vinyl acetate copolymers is reduced and the rheology is modified by the addition of a cross-linking agent to the copolymer. Churma et al teach that in the process, there is essentially no detectable cross-linking in the copolymer and the major affect of the reaction of the cross-linking agent with a copolymer is the reduction of the melt index and modification of rheology of the molten copolymer as it is extruded.